1. Field of the Invention
The present invention relates to the location of a concealed conductor, such as an underground cable or pipe.
2. Summary of the Prior Art
It is well known to detect the path (ie, to locate) a buried cable by applying a recognizable signal to that cable, and detecting the magnetic fields generated by the signal on the cable at remote sites. Such techniques are very well known, and widely used, but all depend on access to the cable at some point to apply the signal thereto. This may not always be possible, particularly when the people carrying out the locating are not employees of the utility to whom the cable belongs. Of course, an electrical cable carrying an alternating mains current will itself generate a magnetic field, which can be detected, but it is impossible to distinguish the field thus generated by one cable from that generated by another. It is therefore not usually possible to use the mains current as a way of locating a specific cable.
If an alternating magnetic field is generated at a particular site, that will induce a current on the cable which can be detected at a remote site due to the magnetic field which the current generates in the cable. Therefore, it is possible to induce a current on a cable without having direct access thereto, and subsequently to detect that cable. However, there are often many cables or metal pipes within range of an induction source and signals will be generated on all those simultaneously, so that detection at a remote site cannot identify which cable or pipe is being detected. It is thus not possible to trace the path of a specific cable or metal pipe. For simplicity, the subsequent discussions will use the term "cable" to refer to the concealed conductor. However, the invention is not limited to the detection of cables, but also metal pipes or other concealed conductors.